1. Field of the Invention
This invention relates to a protection circuit for a switching power amplifier, and more particularly, to a protection circuit for protecting the output stage switching circuit from the destruction due to the overload condition thereof.
2. Description of the Prior Art
Recently, as power amplifiers for an audio signal with high power efficiency characteristics, switching power amplifiers such as power amplifiers using pulse-width modulation technique have been presented.
For a protection circuit for preventing an output stage switching circuit of the power amplifier from falling into the overload condition, there has been conventionally known such one as shown in FIG. 1, for example. FIG. 1 as a whole shows a power amplifier having such protection circuit. In the amplifier shown in FIG. 1, bias voltage sources 1a and 1b, resistors 2a and 2b and power field effect transistors (hereinafter referred to as power FET) 3a and 3b constitute an output stage switching circuit 5 for push-pull amplifying a pulse signal em which is pulse-width modulated in accordance with a signal to be amplified and is applied to a terminal 4. An output pulse signal eo derived from a connection point between collectors of the power FET's is demodulated by a low-pass filter 8 having an inductor 6 and a capacitor 7 into an analogue signal and is applied to a loudspeaker (load) 9.
Resistors 10a and 10b and transistors 11a and 11b constitute a protection circuit of the output stage switching circuit 5, and function so that the resistors 10a and 10b detect the respective drain currents of the power FET's 3a and 3b and that the transistors 11a and 11b become conductive based on the respective voltages across the resistors 10a and 10b when each of the drain currents of the power FET's 3a and 3b exceeds a predetermined magnitude, with the result that the drain currents of the power FET's 3a and 3b are limited not to increase beyond the predetermined magnitude.
The above-mentioned protection circuit, however, requires the use of transistors with high speed switching function as the transistors 11a and 11b, thereby increasing manufacturing cost. In addition, since the loudspeaker 9 with reactance is usually used as a load, it is possible that each of the drain-source voltages (V.sub.DS) of the power FET's 3a and 3b becomes greater than the power supply voltage Vc. Accordingly, it is necessary to set the limitation voltage level to which the drain current levels of the power FET's are limited smaller than the limitation voltage level which is determined based on the power supply voltage Vc when the load has no reactance. The power amplifier above, therefore, is not able to derive a high output power by effectively utilizing the maximum output powers of the power FET's 3a and 3b as much as possible.